The invention relates to starting an internal combustion engine. In particular, the invention pertains to a procedure for starting a compression ignition, free piston internal combustion engine.
A free piston internal combustion engine includes one or more reciprocating pistons located in a combustion cylinder. No crankshaft is used to connect the pistons, coordinate their reciprocation, or establish the compression ratio of a fuel-air mixture in the cylinder. Instead, each piston moves in response to forces produced by combustion of the air-fuel mixture in the combustion cylinder. Pressure produced by combustion in one cylinder can be used to compress an air-fuel charge in another cylinder. Or an actuating system can be used to compress the air-fuel mixture following the expansion stroke. The actuating system may be used also to reciprocate the pistons while starting the engine before combustion of the air-fuel mixture occurs.
Because a free piston engine has no crankshaft connecting the pistons for synchronizing the compression and expansion strokes, a control system is used to synchronize piston reciprocation, compression of the air-fuel mixture and its combustion. Piston displacement, piston velocity, pressure in the combustion chamber, compression ratio, and other engine operating parameters are monitored and controlled by an actuator system, which periodically corrects deviations from the desired coordinated piston movement.
While starting a free piston engine, the pistons are displaced by a starter-actuator system preferably using hydraulic, pneumatic, or electromagnetic actuation. Preferably, electric energy is used to actuate the piston when starting an the engine produces electric power output, and hydraulic or pneumatic energy is used to actuate the piston when the engine produces hydraulic or pneumatic output. This invention of cycling the engine while starting in order to eliminate a vacuum condition in the combustion chamber applies to a free piston diesel engine, compression ignition, spark ignition and homogeneous charge compression ignition (HCCI) combustion engines; however, it will be described with reference to HCCI engine operation. When starting the engine, the intake air has a low temperature, but a large compression ratio of the fuel-air charge in the combustion cylinder is required to produce combustion in a compression ignition engine. Therefore, using conventional engine starting techniques, a large magnitude of energy is required to produce the compression ratio required to start the engine, especially under cold starting conditions.
If the engine pistons are driven entirely by an actuator, a large magnitude of energy is required to compress a mixture of fuel and air in the combustion chamber, particularly in a compression ignition engine that requires a high compression ratio for self-ignition to occur. A technique is required to avoid the need for a large capacity energy source to start the engine, and to ensure that combustion occurs and is sustained under a wide range of ambient operating conditions.